linux-kernel - Re: [Linuxarm] Re: [PATCH for next v1 1/2] gpio: omap: Replace raw_spin_lock_irqsave with raw_spin_lock in omap_gpio_irq

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Date:   Fri, 12 Feb 2021 12:57:08 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
Cc:     Grygorii Strashko <grygorii.strashko@...com>,
        Arnd Bergmann <arnd@...nel.org>,
        luojiaxing <luojiaxing@...wei.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Santosh Shilimkar <ssantosh@...nel.org>,
        Kevin Hilman <khilman@...nel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxarm@...neuler.org" <linuxarm@...neuler.org>
Subject: Re: [Linuxarm] Re: [PATCH for next v1 1/2] gpio: omap: Replace
 raw_spin_lock_irqsave with raw_spin_lock in omap_gpio_irq_handler()

On Fri, Feb 12, 2021 at 10:42:19AM +0000, Song Bao Hua (Barry Song) wrote:
> > From: Grygorii Strashko [mailto:grygorii.strashko@...com]
> > Sent: Friday, February 12, 2021 11:28 PM
> > On 12/02/2021 11:45, Arnd Bergmann wrote:
> > > On Fri, Feb 12, 2021 at 6:05 AM Song Bao Hua (Barry Song)
> > > <song.bao.hua@...ilicon.com> wrote:

> > >>> Note. there is also generic_handle_irq() call inside.
> > >>
> > >> So generic_handle_irq() is not safe to run in thread thus requires
> > >> an interrupt-disabled environment to run? If so, I'd rather this
> > >> irqsave moved into generic_handle_irq() rather than asking everyone
> > >> calling it to do irqsave.
> > >
> > > In a preempt-rt kernel, interrupts are run in task context, so they clearly
> > > should not be called with interrupts disabled, that would defeat the
> > > purpose of making them preemptible.
> > >
> > > generic_handle_irq() does need to run with in_irq()==true though,
> > > but this should be set by the caller of the gpiochip's handler, and
> > > it is not set by raw_spin_lock_irqsave().
> > 
> > It will produce warning from __handle_irq_event_percpu(), as this is IRQ
> > dispatcher
> > and generic_handle_irq() will call one of handle_level_irq or handle_edge_irq.
> > 
> > The history behind this is commit 450fa54cfd66 ("gpio: omap: convert to use
> > generic irq handler").
> > 
> > The resent related discussion:
> > https://lkml.org/lkml/2020/12/5/208
> 
> Ok, second thought. irqsave before generic_handle_irq() won't defeat
> the purpose of preemption too much as the dispatched irq handlers by
> gpiochip will run in their own threads but not in the thread of
> gpiochip's handler.
> 
> so looks like this patch can improve by:
> * move other raw_spin_lock_irqsave to raw_spin_lock;
> * keep the raw_spin_lock_irqsave before generic_handle_irq() to mute
> the warning in genirq.

Isn't the idea of irqsave is to prevent dead lock from the process context when
we get interrupt on the *same* CPU?

-- 
With Best Regards,
Andy Shevchenko